So-called chip carriers, of ceramic or plastics material, are typically rectangular in shape, incorporating one or more semiconductor chips or dies, electrical connections being provided between the chip or die and multiple conductor terminals on the carrier. The terminals on the package or carrier, which may constitute closely spaced tape leads or metallized areas on the outer surface of the package or carrier, are arranged to be secured and electrically connected, e.g. by solder, to an electrically conductive array on the surface of a planar electrically insulating laminate, interconnections to the array being provided by means of electrical conductor elements, e.g. of copper, arranged at one or more planes within the laminate. With advances in technology it has become necessary to accommodate and electrically interconnect an increasing number of circuit or component packages or carriers on printed circuit boards.
Previously, a much used form of assembling and interconnecting electrical components on a printed circuit board involved mounting the components on a first side of the board with terminal leads inserted through holes in the board and secured and electrically connected by a soldering process to printed copper conductors on a second side of the board. It has more recently become known that a significant saving in the area of the board required to accommodate the necessary components can be achieved if the components are attached onto the surface of the printed circuit board, such that there is no insertion of terminal leads on the components into holes in the circuit board. Examples include the transition from the dual-in-line package to either the leaded or leadless chip carrier and from the conventional cylindrical resistor with axial leads to the leadless rectangular chip resistor. These new components occupy significantly less area than the previous components with leads inserted into the board and call for a technique of `surface mounting` on to the printed circuit board. Any holes previously incorporated in the board to accommodate inserted leads and also the annular conductor areas which normally surround such holes are superfluous and may be eliminated to the interest of saving space. In addition to providing important economies, this resultant space saving has permitted a greater density of interconnecting elements or tracks of reduced line width and reduced intervening separation on the circuit board. These features are important in achieving economic interconnection of the required larger number of components and enable component or circuit packages or carriers to be employed each embodying a larger number of terminals than previously. Problems have however arisen in mounting chip carriers onto the surface of conventional electrically insulating laminates used for printed circuit boards that are based on a relatively rigid material, such as glass-filled epoxy. It has been found that when terminals on a chip carrier of ceramic material are soldered, or otherwise bonded, to a conductive array on a major surface of the laminate and the resulting assembly subjected to thermal cycling the difference in thermal expansion between the laminate and the chip carrier results in mechanical strain being set up in the regions where the carrier is secured to the laminate, i.e. at the soldered connections. After a number of cycles the connections may fail and the carrier may even become detached from the circuit board. A further problem may also arise in that any flexing or bending forces applied to a circuit board with carriers or packages thereon will give rise to mechanical strain in the connections between the carriers or packages and the laminate and again the connections may fail.
The surface mounting, on printed circuit boards, of a plurality of circuit or component packages or carriers can give rise to the further problem that with high power, and hence heat, dissipation, and with high component density, it may be necessary to introduce appropriate heat sink or spreader means to allow heat to be conducted from the packages or carriers. In previous rigid printed circuit boards it has been proposed to incorporate, as a heat sink or spreader, one or more thermally conducting metal planes in the vicinity of the surface on which the packages or carriers are mounted. In some cases a layer of elastomer may be provided overlying the metal plane or planes to form an electrically insulating surface for supporting the packages or carriers [see K. Smith: "Multilayer PC Board is Friendly Substrate for both Carrier and TAB Packages", Electronics International, Vol. 55, No. 11, at pp. 92-93] and the necessary array of surface conductor elements. This arrangement has the disadvantage that the packages or carriers are not in intimate contact with the heat sink or spreader. A similar disadvantage arises in a further known arrangement [see W. B. Archey, IBM Technical Disclosure Bulletin Vol. 19 , No. 2 at page 412 (July 1976)] involving a rigid circuit board and where a cooling plate provided with studs which are coated with a metal loaded elastomer is brought into contact with heat dissipating components which are all of necessity of the same height and have terminal leads extending through holes in the circuit board. Such a metal-loaded elastomer, which is necessary to ensure contact with the components, is of lower thermal conductivity than the cooling plate and therefore results in reduced thermal efficiency. It is also known to fabricate the thermally conducting plane integrally with the circuit board but such that it provides for a direct thermal conducting path to the packages or carriers. However, with these previous arrangements the incorporation of the heat sink or spreader facility into the circuit board during the fabrication of the latter increases the complexity of the manufacturing process. It also imposes a limitation on the thickness of the metal plane which can be adopted for use as the heat sink or spreader with the result that a much less than ideal thickness may have to be employed, with consequent reduced efficiency of operation.